The present invention relates to the improvement of a loudspeaker reproduction apparatus which is installed in a vehicle and has a sound volume balance control circuit which regulates the levels of audio signals to be applied to a plurality of loudspeakers disposed in front and rear positions in the vehicle.
A typical conventional loudspeaker reproduction apparatus installed in a vehicle includes a total of four loudspeakers, two loudspeakers disposed either at the sides of the instrument panel of the vehicle or in the right and left front doors thereof, and other two loudspeakers disposed at both sides of the pit of the vehicle, i.e., the deck behind the rear seat and beneath the rear window. The four-loudspeaker system is provided with a fader control circuit which regulates the balance of sound volumes of the front and the rear loudspeakers.
FIG. 4 shows a wiring diagram of an example of such a conventional loudspeaker reproduction apparatus which comprises input terminals 1R and 1L for right-channel and left-channel audio signals, respectively, variable resistors 2 and 3, having sliding terminals 2' and 3' which are ganged for operation in conjunction with each other, power amplifiers 4, 5, 6 and 7, a left front loudspeaker 8L, a right front loudspeaker 8R, a left rear loudspeaker 9L, a right rear loudspeaker 9R, and resistors R.sub.1, R.sub.2, R.sub.3 and R.sub.4. The left-channel audio signal sent to the input terminal 1L is applied to the loudspeakers 8L and 9L through the resistors R.sub.1 and R.sub.2 and the power amplifiers 4 and 6 and is grounded through the variable resistor 2. The right-channel audio signal sent to the input terminal 1R is applied to the loudspeaker 8R and 9R through the resistors R.sub.3 and R.sub.4 and the power amplifiers 5 and 7 and is grounded through the variable resistor 3.
The variable resistors 2 and 3, which operate in conjunction with each other, constitute a front and rear sound volume balance control circuit. The sliding terminals 2' and 3' of the variable resistors 2 and 3 are moved to regulate the balance of sound volumes of the front loudspeakers 8L and 8R and the rear loudspeakers 9L and 9R. When the sliding terminals 2' and 3' are moved in a direction shown by an arrow R in FIG. 4, the outputs from the rear loudspeakers 9L and 9R are increased and those from the front loudspeakers 8L and 8R are decreased. When the sliding terminals 2' and 3' are moved in another direction shown by an arrow F in FIG. 4, the outputs from the front loudspeakers 8L and 8R are increased and those from the rear loudspeakers 9L and 9R are decreased.
Since the rear loudspeakers 9L and 9R are disposed in a rear pit or the like which has a relatively large installation space for them, the diameter of each of the rear loudspeakers to be installed in the space can be made relatively large, e.g., 16 cm to 20 cm. The trunk space of the vehicle can be utilized as an acoustic cavity behind the rear loudspeakers to reproduce a sound of low pitch.
On the other hand, the diameter of each of the front loudspeakers 8L and 8R cannot be made larger than about 10 cm, because the installation space for each of them is limited. Further, the battle effect for the front loudspeakers 8L and 8R is often insufficient. For these reasons, it is difficult to reproduce a sufficient low pitch sound from the front loudspeakers 8L and 8R.
Since the levels of the audio signals to be applied to the loudspeakers 8L, 8R, 9L and 9R are regulated by the sound volume balance control circuit without considering the difference between the low-pitch sound reproduction characteristic of the front loudspeaker 8L and 8R and that of the rear loudspeakers 9R and 9L, there is a problem that the frequency balance of reproduced sound is likely to change when the volume balance is adjusted. Particularly when the sliding terminals 2' and 3' of the variable resistors 2 and 3 are moved in the direction shown by the arrow F in FIG. 4, the audio signals are only slightly applied to the rear loudspeakers 9L and 9R capable of reproducing relatively sufficient low-pitch sounds, so that the low-frequency portion of the entire sound reproduced by all the loudspeakers become quite insufficient.
In order to solve the above-mentioned problem, another conventional loudspeaker reproduction apparatus shown in FIG. 5 has been proposed. The difference of the conventional loudspeaker reproduction apparatus shown in FIG. 5 from that shown in FIG. 4 is that capacitors C.sub.1 and C.sub.2 are connected between ground and the sliding terminals 2' and 3' of variable resistors 2 and 3. The same reference symbols in FIGS. 4 and 5 otherwise designate the same components. In the loudspeaker reproduction apparatus shown in FIG. 5, the capacitors C.sub.1 and C.sub.2 are connected between ground and the sliding terminals 2' and 3' of the variable resistors 2 and 3 so that the low-frequency components of audio signals sent to input terminals 1L and 1R are always applied both to front loudspeakers 8L and 8R and to rear loudspeakers 9L and 9R to solve the above-mentioned problem of the loudspeaker reproduction apparatus shown in FIG. 4. In other words, low-pitch sounds which give less of a feeling of direction are always reproduced by both the front loudspeakers 8L and 8R and the rear loudspeakers 9L and 9R to prevent the low-pitch portion of the entire reproduced sound from becoming insufficient.
However, the loudspeaker reproduction apparatus shown in FIG. 5 also has a problem that, when the sliding terminals 2' and 3' of the variable resistors 2 and 3 are moved in a direction shown by an arrow R in FIG. 5, only the low-frequency components of the audio signals are applied to the front loudspeakers 8L and 8R of poorer low-pitch sound reproduction characteristic, so that the distortions of the sounds reproduced by the front loudspeakers are more audible. Since the front loudspeakers 8L and 8R are generally located near the listener, the frequency cut off by the capacitors C.sub.1 and C.sub.2 needs to be made considerably low in order to prevent the distortions of the sound from becoming more audible when the sliding terminals 2' and 3' are moved in the direction shown by the arrow R. However, if the cut-off frequency is made considerably low, it is difficult to cause the rear loudspeakers 9L and 9R to compensate for the insufficiency of the low-pitch sounds reproduced by the front loudspeakers 8L and 8R when the sliding terminals 2' and 3' are moved in the reverse direction shown by an arrow F.
The present invention has been made in order to solve the above-mentioned problems.